1. Field of the Invention
The present invention relates in general to a digital automatic level control system that employs digital elements for monitoring and controlling a signal parameter, such as power, voltage, etc. The system can control the levels or gains of multiple signals or channels either individually or in groups. Through use of information relating to the signal parameter's past, present and future levels, the system can respond accurately to quickly varying signals, such as bursty signals.
2. Description of the Background Art
There are numerous applications where power or other parameter levels of signals must be adjusted to facilitate processing. For example, in satellite communications systems, signals transmitted from sources in different areas of a satellite's uplink beam where atmospheric and antenna gain may vary, result in received power at the satellite that varies on a user-by-user basis. In addition, suboptimal tuning of each user's transmit antenna may increase the variation in received power at the satellite. Furthermore, users transmitting from discrete and disjoint beams produce received power at the satellite that varies through an even wider range of values.
One problem with receiving signals that vary in power over a wide range is that devices in the satellites often cannot work efficiently with such signals. For example, one known satellite communications system employs a digital transponder to combine, in the digital domain, user signals received from different uplink beams onto a single downlink beam, powered by a single TWTA, SSPA or other type of power amplifier (PA). All realizable amplifiers produce a nonlinear input power-to-output power response as input power and output power are increased. This nonlinearity produces an output signal that diverges from the input signal in a mean squared sense. To compensate for this undesirable effect, an engineering trade must be made, considering the maximum and minimum input powers, to set the gain of the PA in order that these nonlinearities are not produced, a process termed “backing off.” If the input power to the PA varies by a great amount, the PA must be backed off by a large value, resulting in reduced PA efficiency. Thus, it is desirable to have an input signal to the PA that has as small a power variation as possible. In the context of the digital transponder, if received user signals from different uplink beams are combined to be transmitted on a single downlink beam with a single PA, and those user signals vary in power by a great amount, it is beneficial to the efficiency of the digital transponder to have a normalization scheme to level each user's received power in the digital transponder.
Current automatic level control (ALC) and automatic gain control (AGC) schemes perform power leveling using analog input/output signals. This is disadvantageous, because classical control theory must be employed, resulting in a delay in reaction time of the ALC, and/or sub-optimal leveling. As a result, such schemes do not work well with certain types of rapidly varying signals, such as are employed in burst communications schemes, for example. In addition, analog based schemes cannot be efficiently adapted for use with digital signals such as are employed in the aforementioned digital transponder, for example. A need therefore exists for a digital automatic level control system that can be readily adapted for accurately leveling all types of digital signals, including rapidly varying signals, such as bursty signals.